The different types of communication signals are typically classified according to modulation type. Each of the various forms of modulation has its own set of advantages and disadvantages relative to a specific application for which it will be used. Some factors to consider in choosing a particular form of modulation include bandwidth, power consumption requirements, and the potential for signal propagation errors and recovery of the original information. Low power consumption is particularly sought when using capacitively-loaded transmission lines. For digital data, whether the modulated signal is self-clocking or requires a separate reference clock may be important. The relative simplicity or complexity of the modulating and demodulating circuits may also be a factor in the decision to chose a particular modulation form.
Each type of signal modulation has specialized decoder circuitry for performing the demodulation and data recovery. For example, U.S. Pat. No. 5,614,861 to Harada describes a system for demodulation of a phase-modulated signal, in particular a quad phase-shift keyed (QPSK) signal, that employs a set of phase detectors with one input of each phase detector receiving the signal to be demodulated. At least one other input receives a reference clock of specified phase delay, different for each phase detector, from a phase-locked loop (PLL) controlled clock generation circuit. Results of the phase detection are then input into data generation circuitry comprising logic gates for converting the detection results into a pair of data bits.
A decoder circuit for a low power, high bandwidth, pulse width type of signal modulation would be desirable.